U.S. patent number 10,749,171 [Application Number 15/548,549] was granted by the patent office on 2020-08-18 for method for the preparation of anodes for lithium batteries.
The grantee listed for this patent is Zhongwei Chen, Fathy Mohamed Hassan, Aiping Yu. Invention is credited to Zhongwei Chen, Fathy Mohamed Hassan, Aiping Yu.
View All Diagrams
United States Patent |
10,749,171 |
Chen , et al. |
August 18, 2020 |
Method for the preparation of anodes for lithium batteries
Abstract
A method for preparing an electrode for use in lithium batteries
and the resulting electrodes are described The method comprises
coating a slurry of silicon, sulfur doped graphene and
polyacrylonitrile on a current collector followed by sluggish heat
treatment.
Inventors: |
Chen; Zhongwei (Waterloo,
CA), Yu; Aiping (Waterloo, CA), Hassan;
Fathy Mohamed (Waterloo, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Chen; Zhongwei
Yu; Aiping
Hassan; Fathy Mohamed |
Waterloo
Waterloo
Waterloo |
N/A
N/A
N/A |
CA
CA
CA |
|
|
Family
ID: |
56563273 |
Appl.
No.: |
15/548,549 |
Filed: |
February 5, 2016 |
PCT
Filed: |
February 05, 2016 |
PCT No.: |
PCT/CA2016/050108 |
371(c)(1),(2),(4) Date: |
August 03, 2017 |
PCT
Pub. No.: |
WO2016/123718 |
PCT
Pub. Date: |
August 11, 2016 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180013138 A1 |
Jan 11, 2018 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
62176004 |
Feb 6, 2015 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
4/667 (20130101); H01M 10/0525 (20130101); H01M
4/1395 (20130101); H01M 4/583 (20130101); H01M
4/663 (20130101); H01M 4/622 (20130101); H01M
4/133 (20130101); H01M 4/624 (20130101); H01M
4/1397 (20130101); H01M 4/625 (20130101); H01M
4/136 (20130101); H01M 4/1393 (20130101); H01M
4/134 (20130101); H01M 4/62 (20130101); H01M
4/668 (20130101); H01M 10/052 (20130101); H01M
4/366 (20130101); H01M 4/0471 (20130101); Y02E
60/10 (20130101); H01M 2004/027 (20130101) |
Current International
Class: |
H01M
4/36 (20060101); H01M 4/1395 (20100101); H01M
4/133 (20100101); H01M 4/04 (20060101); H01M
4/1393 (20100101); H01M 4/1397 (20100101); H01M
4/136 (20100101); H01M 4/62 (20060101); H01M
4/134 (20100101); H01M 10/052 (20100101); H01M
10/0525 (20100101); H01M 4/66 (20060101); H01M
4/583 (20100101); H01M 4/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
104269516 |
|
Jan 2015 |
|
CN |
|
104319372 |
|
Jan 2015 |
|
CN |
|
104332616 |
|
Feb 2015 |
|
CN |
|
20020068930 |
|
Aug 2002 |
|
KR |
|
Other References
English machine translation of Yoon et al. (KR 20020068930 A)
(Year: 2002). cited by examiner .
English machine translation of Wang et al. (CN 104319372 A) (Year:
2015). cited by examiner .
International Search Report and Written Opinion, dated Jun. 13,
2016, ISA/CA. cited by applicant .
Hassan et al., "Evidence of Covalent Synergy in
Silicon-Sulfur-Graphene Yielding Highly Efficient and Long-life
Lithium Batteries", Nature Communications, vol. 6, Article No.
8597, Oct. 26, 2015. cited by applicant .
Liu et al., "Three-Dimensional Hierarchical Ternary Nanostructures
for High-Performance Li-Ion Battery Anodes", Nano Letters, 2013,
vol. 13 (7), pp. 3414-3419. cited by applicant .
Wang et al., "Carbon-sulfur composites for Li--S batteries: status
and prospects", Journal of Materials Chemistry A, 2013 vol. 1, pp.
9382-9394. cited by applicant .
De Guzman, Rhet C. et al., "Effects of graphene and carbon coating
modifications on electrochemical performance of silicon
nanoparticle/graphene composite anode", Journal of Power Sources
246 (2014), pp. 335-345. cited by applicant .
Piper, Daniela Molina et al., "Conformal Coatings of Cyclized-PAN
for Mechanically Resilient Si nano-Composite Anodes", Advanced
Energy Materials (2013), pp. 697-702. cited by applicant .
Yun, Young Soo et al., "Effects of sulfur doping on graphene-based
nanosheets for use as anode materials in lithium-ion batteries",
Journal of Power Sources 262 (2014), pp. 79-85. cited by
applicant.
|
Primary Examiner: Fraser; Stewart A
Attorney, Agent or Firm: Holzer Patel Drennan
Claims
We claim:
1. A method for preparing an anode comprising: combining silicon
(Si), sulfur doped graphene (SG) and polyacrylonitrile (PAN) to
form a slurry; coating the slurry on a current collector; and
subjecting the current collector coated with the slurry to a
sluggish heat treatment (SHT).
2. The method of claim 1 wherein the SHT comprises gradual heating
of 1-12.degree. C. per minute to a peak temperature in the range of
about 300.degree. C. to about 700.degree. C.
3. The method of claim 2 wherein the peak temperature is about
500.degree. C.
4. The method of claim 2 wherein the peak temperature is about
450.degree. C.
5. The method of claim 1 wherein the SHT comprises heating at a
rate in the range of about 1-12.degree. C. per minute.
6. The method of claim 1 further comprising a cooling step
following the SHT wherein the cooling step comprises cooling at a
rate in the range of about 1-12.degree. C. per minute.
7. The method of claim 1 wherein the Si is one or more of Si
powder, Si nanowire, Si nanoparticle (SiNP), Si sol particle, Si
rod, and a combination thereof.
8. The method of claim 1 wherein the Si is Si nanoparticle
(SiNP).
9. The method of claim 1 wherein the current collector is a copper
current collector.
10. The method of claim 1 wherein the SHT comprises heating at a
rate of 5.degree. C. per minute and to a temperature of 450.degree.
C. sufficient to cause cyclization of the PAN.
11. The method of claim 1 wherein a catalyst is added to catalyze
cyclization of the PAN.
12. The method of claim 11 wherein the catalyst is graphitic oxide
(GO).
13. The method of claim 1 wherein a solvent is added when forming
the slurry.
14. The method of claim 13 wherein the solvent is dimethylformamide
(DMF).
15. The method of claim 1 wherein the coating is dried before the
SHT.
16. The method of claim 1 wherein the coating on the current
collector comprises the Si bound to the SG and at least partially
encompassed in a shell of cyclized PAN (c-PAN).
Description
FIELD
The present invention relates to a method for the preparation of
electrodes for lithium batteries and to the anodes prepared by said
method. More particularly it relates to a method for preparing an
anodes comprising silicon particles bound to sulfur-doped graphene
combined with polyacrylonitrile.
BACKGROUND
The success of high performance portable electronics and hybrid (or
electric) vehicles strongly depends on further technological
progress of commercially available rechargeable batteries.
Lithium-ion batteries (LIBs) are considered the most likely energy
storage configuration to satisfy these demands. However, this
requires significant advances in terms of power density, energy
density, cycle life and safety, as well as lower production costs.
Current LIBs utilize graphite anodes where energy is stored by
intercalating lithium into the graphite layers. This arrangement
while commercially successful can only deliver a maximum
theoretical capacity of 370 mAhg-1, (Shang W. J.; A review of the
electrochemical performance of alloy anodes for lithium ion
batteries, J. Power Sources 196, 13-24 (2011)). Incorporating
additional components offers the potential to dramatically improve
this capacity. For example silicon can provide up to 4200 mAhg-1,
in theory, corresponding to the following alloying reaction: 4.4
Li+Si.fwdarw.Li4.4Si (1) While Si-based composites offer immense
promise as new generation anode materials, extreme changes in
volume during lithiation and delithiation lead to structural
degradation and loss of performance over time that impedes their
practical application.
Several journal articles as well as patents are concerned with the
improving performance and cycle stability of silicon. Magasinski et
al. (Nature Material, 9 (2010) 353-358) prepared silicon
nanoparticles by silane decomposition onto annealed carbon-black
dendritic particles and followed by coating with carbon in a
chemical vapour deposition (CVD) process. This paper describes
reversible capacities over five times higher than that of the
state-of-the-art anodes (1950 mA h g-1) and stable performance. Cui
et al. (Nature Nanotechnology, 3 (2008) 31-35) prepared high
performance anodes based on silicon nanowires. They prepared the
silicon nanowires in a CVD process using the vapour-liquid-solid
(VLS) method with gold as a catalyst. The paper describes achieving
the theoretical capacity of the silicon anodes and maintained a
discharge capacity close to 75% of the maximum. However, this
process employs costs catalyst material. Kim et al. (Nano letters,
8, (2008) 3688-3691) prepared a Si core and carbon shell structure
by using SBA-15 mesoporous silica material as a template. They
reached a first charge capacity of 3163 mA h/g with a coulombic
efficiency of 86% at a rate of 600 mA/g, and they retained 87% of
their capacity after 80 cycles. However, when they increased the
rate capability to 6 A/g the capacity decreased to 78%. In US
2005/0031957 A1, silicon microparticles were mixed with an
electrochemically inactive phase that includes an intermetallic
compound that is formed of at least two metals and a solid solution
yielding a composition of Si55Al30Fe15 (for example). Even though,
these electrodes showed improved cycle stability, they had a great
loss in specific capacity due to the inclusion of inactive
components in the electrode. US 2009/0130562, describes coated
silicon nanoparticles with carbon and their use as anode material.
The composite material comprising silicon, carbon and graphite
showed a capacity of around 900 mAh/g for almost 5 cycles. US
2010/0062338 A1, describes the use of silicon nanoparticles as an
active material and an elastomeric binder to bind the silicon
nanoparticles as well as the addition of conductive material such
as super P or graphite. In this patent the author claims that these
electrode additives improved cycle stability of the battery;
however, they did not disclose specific performance results. In US
2012/0121977 A1, the inventors describe an interfacial layer around
the silicon nanoparticle. The layer has good electron conductivity,
elasticity and adhesion. This layer is formed of a monomer and a
polymer with several functional groups. The capacity is about 400
mAh/g and increasing with the cycle number up to a maximum at about
1000 mAh/g at about 100 cycles then decay back during the next 100
cycles reaching 700 mAh/g at the 200th cycle. In US 2012/0129054,
the inventors used silicon nanowires with or without carbon coating
and also they claim the addition of diallyl pyrocarbonate to the
electrolyte during the battery fabrication.
US2014/0186701 to Zhang et al. describes a composite anode prepared
by electrophoretic deposition (EDP) of a suspension comprising one
or more of silicon, carbon and a current collector onto a copper
current collector and allowing the deposited material to dry on the
carbon substrate.
Despite the various approaches proposed in the literature, there is
no approach to directly use commercially available silica
nanoparticles with affordable, economic and environmentally safe
treatment methods for fabrication of lithium ion batteries. There
remains a need for a solution to prevent the loss in specific
capacity due to addition of inactive materials needed to enhance
stability. There further remains a need for a method to prepare
anode that are stable and provide sufficiently high performance at
an acceptable cost.
SUMMARY
In one aspect of the invention there is a provided a method for
preparing an electrode comprising:
combining silicon (Si) sulfur doped graphene (SG) and
polyacrylonitrile (PAN) to form a slurry,
coating the slurry on a current collector and
subjecting the coated current collector to a sluggish heat
treatment (SHT).
In an embodiment of the method the sluggish heat treatment
comprises heating at a rate and to a temperature sufficient to
cause cyclization of PAN.
In a further aspect of the invention there is provided an anode
prepared by the method comprising:
combining silicon (Si) sulfur doped graphene (SG) and
polyacrylonitrile (PAN) to form a slurry,
coating the slurry on a current collector and
subjecting the coated current collector to a sluggish heat
treatment (SHT).
In still a further aspect of the invention there is provided a
anode comprising a current collector coated with a composition
comprising silicon (Si), sulfur doped graphene (SG) and cyclized
polyacrylonitrile (c-PAN).
In a further aspect of the invention there is provided a lithium
ion battery comprising at least one cathode and at least one anode
wherein the anode is as defined herein.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described by way of
example only with reference to the accompanying drawing
wherein:
FIG. 1: a) is a schematic of flash thermal shock to convert
graphitic oxide (GO) to sulfur-doped graphene (SG) and b) is a
scanning electron microscopy (SEM) image of SG;
FIG. 2: is a Schematic of electrode process design wherein a) shows
components mixing under ultrasonic irradiation b) is an optical
image of the as-fabricated electrode made of SiNP, SG and PAN, c)
shows the electrode after SHT, d) is a schematic of the atomic
scale structure of the electrode and e) is a TEM image of the
electrode after SHT;
FIG. 3: is a TEM characterization of the electrode a) is a
HAADF-STEM image of the SG-Si electrode, b) is a higher
magnification HAADF-STEM image of SG-Si, and c) shows EELS mapping
of the elements Si and S, with each pixel representing
3.4.times.3.4 nm, d) HAADF-STEM images zooming in on interconnected
SiNPs in the SG-Si electrode, e) is a regular TEM image of the
image in d, f) is an HRTEM image of a SiNP with carbon shell and
graphene. Scale bars is 100 nm in (a, b, d, and e), 20 nm in (c),
and 5 nm in (f);
FIG. 4: is an element analysis in electrode showing electron energy
loss spectrum for SG-Si electrode after sluggish heat treatment
(SHT);
FIG. 5: a) Raman spectra of PAN alone before and after SHT b) Raman
spectra of SG-Si-PAN before and after SHT;
FIG. 6: Shows the structure changes of PAN before and after SHT
wherein (a) is a differential scanning calorimetry (DSC) for
polyacrylonitrile (PAN) in nitrogen showing a characteristic peak
at .about.300.degree. C., which corresponds to PAN cyclization (as
shown in the proposed schematic (d)); (b) shows a thermogravimetric
analysis for PAN in both air and in nitrogen. During cyclization in
nitrogen there is more loss in mass which reveals it is more
efficient than in air. By cyclization PAN loses .about.20% of its
mass, (c) Nitrogen high resolution XPS of SG-Si-PAN (before SHT),
and SG-Si--C-PAN (after SHT) (d) schematic showing proposed
cyclization of PAN;
FIG. 7: graph showing electrochemical impedance for a coin cell
fabricated using PAN-coated copper foil vs. lithium, (same method
of cell testing as described herein below) showing that both the
electrode series resistance and the charge transfer resistance have
been decreased after the sluggish heat treatment;
FIG. 8: Morphology of the electrode (a) is a TEM image of SG-Si
electrode material, (b-f) show the corresponding EDX mapping of the
elements carbon, oxygen, silicon, sulfur, and nitrogen,
respectively, and (g) overlaid map of carbon, silicon, and
sulfur.
FIG. 9: Electrode material characterization for SG-Si a) is an XPS
survey spectra confirming the elements Si, S, C, N and O, b) is a
high-resolution XPS spectra of carbon in SG-Si, c) is a high
resolution XPS of Si 2p in SG-Si, d) is a high-resolution XPS
spectra of sulfur in pure SG, e) is a high resolution XPS of sulfur
in 1) electrode material made of elemental S, SiNP and PAN, 2)
electrode material of (1) after being subjected to SHT, 3)
electrode material made of SG, SiNP and PAN, and 4) electrode
material of (3) after being subjected to SHT. (au arbitrary
unit);
FIG. 10: Sulfur distribution on SG nanosheet (a) is an STEM-HAADF
of a SG nanosheet in a micron size, (b) and (c) are the EDX mapping
for sulfur and carbon, respectively; (d) is the electron energy
loss spectroscopy (EELS) mapping and (e) represent the EELS mapping
of sulfur in pixilated grey color, each pixel represent 10.times.10
nm. The figure clearly shows the doping with sulfur in the bulk of
SG nanosheet as well as on the edges;
FIG. 11: Morphology of SG-Si-PAN electrode (a) shows the
as-prepared electrode after drying, (b) shows the electrode after
sluggish heat treatment, and (c) shows the electrode extracted from
a coin cell which was cycled for 100 cycles;
FIG. 12: Porosity in the electrode is shown by comparison of pore
size distribution for the SG-Si electrode before and after SHT. The
pore volume increases after SHT which provide void space that
compensate the volume expansion of Si during lithiation;
FIG. 13: Electrochemical performance of SG-Si a) shows the voltage
profile of SG-Si anode at 0.1 A g.sup.-1, b) The corresponding
cycle stability, c) cyclic voltammogram curves of the SG-Si coin
cell, d) rate capability of SG-Si anode followed by cycle stability
at 2Ag.sup.-1, e) rate capability of G-Si anode followed by cycle
stability at 2Ag.sup.-1, f) rate capability of Si-PAN anode
followed by cycle stability at 2Ag.sup.-1, and g) a pie chart
showing the relative contribution of the electrode materials for
the capacity seen in (d); h) is a graph showing the voltage profile
for a full cell battery based on SG-Si anode and LiCoO.sub.2
cathode, and i) is a graph showing the corresponding cycle
stability at 1 Ag-1 with respect to SG-Si, the inset is the first 5
cycles at 0.1Ag.sup.-1;
FIG. 14: Cyclic voltammogram curves of G-Si anode material in coin
cell;
FIG. 15: Cycle stability of a reference cell was demonstrated using
a cell fabricated using SiNP (60%), PVDF (polyvinylidene fluoride)
(20%) as binder, and super P (20%) as conducting carbon, the
performance was tested at 0.1Ag.sup.-1;
FIG. 16: Reference battery testing (a) SG-PAN, and (b) only c-PAN,
after being subjected to SHT treatment. The cells were tested at
0.1 A g.sup.-1 then continued at 2 A g.sup.-1. The SG-PAN provided
reversible capacity of .about.250 mAh g.sup.-1 and the c-PAN
provide .about.25 mAh g.sup.-1;
FIG. 17: Cycling performance for reference batteries. These were
fabricated using SG+SiNP+PVDF with no SHT treatment. (a) The cell
subjected to rate capability at different current then continued at
2 A g.sup.-1 (b) The cell was tested at 0.1 A g.sup.-1 for 5 cycles
then continued at 2 A g.sup.-1 for the rest;
FIG. 18: Cycling performance for reference batteries. These were
fabricated using SiNP+Graphene oxide+PAN with SHT treatment. (a)
The cell subjected to rate capability at different current then
continued at 2 A g.sup.-1. (b) The cell was tested at 0.1 A
g.sup.-1 for 5 cycles then continued at 2 A g.sup.-1 for the
rest;
FIG. 19: Volumetric Capacity for SG-Si-c-PAN electrode for the cell
performance with data shown in FIG. 5b;
FIG. 20: Further battery performance for SG-Si-c-PAN electrode with
ratio of 40:30:30, respectively. (a) shows the cell cycled at 0.1 A
g.sup.-1 for conditioning then continued at 1 A g.sup.-1. (b) The
cell started conditioning cycles then continued with rate
capability at different currents then continued cycling at 2 A
g.sup.-1. The capacity measured here is per mass of silicon and
SG;
FIG. 21: Characterization of SG-Si electrode material after cycling
for 2250 cycles. a) HAADF-STEM image of the SG-Si electrode after
cycling, b-d) the elements mapping by EELS for the area marked in
image. Scale bar in (a) is 100 nm, and in (b-d) is 10 nm. Each
pixel in Figs b-d represents 3.4.times.3.4 nm. (e) A schematic to
explain the structure change in the electrode before and after
cycling. Before battery cycling SiNP are dispersed, and bond with S
on surface of SG with c-PAN further connect the SiNP with SG. After
battery cycling, the SiNP change to amorphous structure and spread
and confine in the crinkles of SG; f) image of SG-Si electrode
before cycling;
FIG. 22: After cycling characterization of SG-Si. (a) STEM image of
SG-Si electrode material after being cycled for 2275 cycles of
charge discharge, b-f) the corresponding EDX mapping of the
elements carbon, oxygen, silicon, sulfur, and nitrogen,
respectively;
FIG. 23: After cycling characterization of G-Si. The figure shows
HAADF-STEM image of the G-Si electrode after cycling for 800 cycles
as shown in FIG. 5e, it shows the agglomeration of Si which
explains the capacity fading;
FIG. 24: The optimized geometry of H passivated graphene (G). Top
view (top) and side view (bottom). C atoms are colored grey, H
atoms white. Bond length is in angstrom;
FIG. 25: The optimized geometry of sulfur-doped graphene (S-G). top
view (top) and side view (bottom). C atoms are colored grey, H
atoms are white, S atom is yellow. Bond lengths are in
angstrom;
FIG. 26: DFT quantum calculations for G-Si and SG-Si systems.
Geometries and binding energy (BE) of the stable Si adsorption
configurations on a): graphene, referred as G-Si; b) and c) on
sulfur doped graphene, referred as SG-Si(A) and SG-Si(B),
respectively, C atoms are colored grey, H atoms white, S atom
yellow, Si atom brown. Some of the important atoms were labeled,
and they correspond to the atoms in Table 1, and d and e) The DFT
calculated binding energy (BE) of the stable cluster of 9 Si atoms
adsorption configurations to SG with different defect
configurations. The bond lengths shown in the figure are in
angstroms;
FIG. 27: Geometries and bonding energy (BE) of the stable Si.sub.4
cluster adsorption configurations. (a) On graphene, (b) On sulfur
doped graphene. C atoms are colored grey, H atoms are white, S atom
are light grey, Si atoms are grey;
FIG. 28: Projected density of states (PDOS). The PDOS for Si atom
and the individual C atoms involved in a) Si adsorption on
graphene, G-Si, and (b-d) Si adsorption on sulfur doped graphene,
SG-Si(B);
FIG. 29: Li adsorption and transition state. The figure quantifies
Li diffusion barrier for a) G-Si and b) SG-Si.
DETAILED DESCRIPTION
It has been found that electrodes and in particular anodes for
lithium ion batteries can be prepared by a method of coating a
slurry comprising Silicon (Si), sulfur doped graphite (SG) and
polyacrylonitirile (PAN) onto a current collector allowing the
coating to dry, followed by heating under conditions of "sluggish
heat treatment" (SHT).
Silicon (Si)
The Si may be in the form of Si powder, Si nanowire, Si
nanoparticle (SiNP) Si sol particle or Si rod or a combination
thereof. Various forms of Si would be known to one of skill in the
art and may be used. The Si may be used in various commercially
available forms.
Sulfur Doped Graphite (SG)
Sulfur doped graphene can be prepared from graphitic oxide (GO) by
a modified Hummer's method..sup.1-3 In one example 100 mg of GO was
mixed with 100 mg of phenyl disulphide by grinding. The materials
were then loaded into a tube furnace and kept outside the heating
zone until the furnace temperature reached 1000.degree. C. The
sample was then slid into the heating zone where it remained for 30
min. under argon protection followed by cooling to room
temperature. FIG. 1a is a schematic depicting a flash thermal shock
to convert graphitic oxide (GO) into sulfur doped graphene (SG). An
SEM image of SG is shown in FIG. 1b. (Graphene without sulfur is
used for comparative experiments described herein and was prepared
under identical conditions to sulfur doped graphene but without
phenyl disulphide.)
Polyacrylonitrile (PAN)
PAN is a synthetic resin prepared by the polymerization of
acrylonitrile. It is a hard, rigid thermoplastic material that is
resistant to most solvents and chemicals, slow to burn, and of low
permeability to gases. Under conditions of sluggish heat treatment
(SHT) PAN is converted to cyclized PAN (c-PAN) and becomes
conductive. While other conducting agents such as PANI may be used,
c-PAN is advantageous in that it is a relatively low cost option.
C-PAN formed under condition of SHT in the present method has also
been found to be capable of stabilizing a battery in use for more
than 200 cycles.
Current Collector
Various current collector materials will be known to one of skill
in the art and may be used. In one embodiment the current collector
is a copper current collector which may take the form of a Cu grid,
Cu foil or Cu foam.
A slurry of Si, SG and PAN can be prepared by a variety of suitable
methods which would be known to one of skill in the art. For
example, the slurry may be formed by combining the reagents in a
solvent. Suitable solvents will be known to one of skill in the art
and may include for example one or more of DMF and pyridinium
benzylchloride. The mixture may then be subjected to a mixing step.
Suitable mixing conditions will be known or may be determined by
one of skill in the art and may include ultrasonic radiation or
magnetic stirring or a combination thereof. Other suitable methods
may include ball milling. In a particular embodiment the reagents
are mixed by alternating ultrasonic radiation and magnetic stirring
(1 hour each, three times).
The slurry is then coated, cast or deposited on to a current
collector. Various methods will be known to one of skill in the art
for coating the slurry onto the current collector such as doctor
blade, spin coating or screen printing.
The slurry is allowed to dry on the substrate. In a particular
embodiment drying may be accelerated by heating in a convention
oven at approximately 353K for about 1 hour and then in a vacuum
oven at 363 K overnight.
The material is then subjected to SHT. SHT generally refers to a
process of slow heating to a peak temperature holding at the peak
temperature for a duration of time and slowly cooling. In one
embodiment the peak temperature will be in the range of about
300.degree. C. to about 700.degree. C. In a further embodiment the
peak temperature is between about 400.degree. C. to about
600.degree. C. In a particular embodiment the peak temperature is
about 450.degree. C. to about 550.degree. C., and further
embodiments the peak temperature is about 500.degree. C. In one
embodiment the rate of heating is about 1 to about 12 degrees
Celsius per minute. In a further embodiment the rate of cooling is
about 1 to about 12 degrees Celsius per minute. In a further aspect
the sluggish heat treatment is conducted under inert gas atmosphere
for example under Nitrogen or Argon atmosphere.
An additive may be included to induce or catalyze cyclization of
the PAN. In a particular embodiment graphitic oxide (GO) may be
included as an additive to induce cyclization of the PAN by
oxidation. In a further embodiments Oxidized carbon nanotubes may
be used as an additive.
The slurry prepared in the first step of the method includes about
40-70 wt % SiNP, about 15-25 wt % SG, about 15-25 wt % PAN and
about 0-5 wt % GO. In one example the slurry includes 60 wt % SiNP,
19 wt % SG, 20 wt % PAN and 1 wt % GO.
EXPERIMENTAL EXAMPLES
Electrode Fabrication
In one example a slurry consisting of 50% of Si--NP, 30% PAN (as a
binder), 19% of SG and 1% GO was prepared in DMF. The slurry was
mixed under ultrasonic radiation. Then it was coated on Cu foil.
The average mass loading of silicon on the electrodes ranged from
0.8-1.5 mg cm.sup.-2. The electrode was dried in a convention oven
at 353 K for 1 hour, followed by drying in a vacuum oven at 363K
overnight.
In a further example the slurry was prepared with 60 wt % SiNP, 19
wt % SG, 20 wt % PAN and 1 wt % GO.
In a further example a reference electrode for comparison was
prepared with 70 wt % SiNP, and 30 wt % PAN.
In a further example a reference electrode for comparison was
prepared use graphene in place of sulfur doped graphene.
A schematic of the electrode fabrication process is shown in FIG.
2. Components mixing under ultrasonic irradiation are shown in a).
The slurry prepared in a) is coated onto an substrate, typically Cu
foil, to provide the as-fabricated electrode of SiNP, SG and PAN as
shown in the optical image b and corresponding schematic (schematic
of the atomic scale structure is shown as d). The electrode is then
treated under sluggish heat treatment (SHT) conditions to provide
the material shown in optical image c) and corresponding schematic.
A transmission electron microscopy (TEM) image of the electrode of
FIG. 2 after SHT is shown in e).
In one embodiment the conditions for SHT include heating to a
temperature of about 450.degree. C. over a period of approximately
2 hours then holding the temperature for 10 minutes followed by
furnace cooling for approximately 2 hours. The SHT treatment may be
performed under inert gas at a flow rate of 100 standard cubic
centimeters per minute (SCCM). In one example the inert gas is
Argon however other inert gases may be used.
Electrochemical Measurements
In order to test the behavior of SG-Si in realistic full cell
setup, a coin cell of SG-Si anode and a commercial LiCoO.sub.2
cathode was assembled. The cell was first charged from OCV to 4.3V
and then cycled between 2.5 to 4.3 V. The first cycle efficiency is
about 84% and the cell was able to give an areal capacity of about
3 mAh cm-2 at a rate of 0.1 Ag-1 with respect to SG-Si mass. When
the rate increased 10 times to 1 Ag-1 the capacity decreased to 0.9
mA cm-2 or .about.800 mAh g-1 (SG-Si) and remains almost stable
with minimum capacity loss for up to 100 cycles.
To test the electrodes, 2032-type coin cells were assembled in an
argon filled glovebox using Celgard 2500 membrane as the separator
lithium foils as the counter electrodes, 1M LiPF.sub.6 in a 3:7
(v/v) mixture of (30 wt %) ethylene carbonate and (60 wt %)
dimethyl carbonate with 10 wt % fluorinated ethylene carbonate
(FEC) as the electrolyte. The galvanostatic charge/discharge
measurements were performed on Neware BTS-CT3008 (Neware
Technology, Ltd., Shenzhen, China) at different current densities
and different cut-off voltage ranges. Electrochemical impedance
spectroscopy measurement was conducted on a Princeton Applied
Research VersaSTAT MC potentiostat. The Nyquist plots were recorded
potentiostatically by applying an AC voltage of 10 mV amplitude in
the frequency range of 0.01 to 100K Hz. All electrochemical
measurements were carried out at room temperature.
Material Characterization
The morphologies of the electrode material were imaged using a
transmission electron microscopic (TEM, JEOL 2010F TEM/STEM field
emission microscope) equipped with a large solid angle for
high-X-ray throughput, and a Gatan imaging filter (GIF) for energy
filtered imaging. Thermal Gravimetric Analysis (TGA) and
Differential Scanning calorimetry (DSC) were measured using TA
instrument Q500. The TGA testing was performed in air with a
temperature range of 25.degree. C. to 850.degree. C. and a ramp
rate of 10.degree. C. min.sup.-1. Raman spectroscopy were recorded
using Bruker Senterra device, applying laser with wavelength of 532
nm.
Quantum Mechanical Computational Method
The DFT calculations were carried out using the Amsterdam Density
Functional "ADF" program..sup.4,5 The electron wave functions were
developed on a basis set of numerical atomic orbitals (NAOs) and of
Slater type orbitals (STOs). In addition the triple polarization
(TZP) basis of Slater-type orbitals was utilized. We used PBE-D3 to
perform the calculations.sup.6 where the generalized gradient
approximation (GGA) for the exchange and correlation energy terms
is used. This explicitly takes into account the dispersion
correction. This is a widely used function for catalysis
applications and can produce reliable energetics on graphene
systems..sup.7,8
Morphology and Structure of the Electrode
The high angle annular dark field (HAADF) scanning transmission
electron microscope (STEM) image in FIG. 3a shows a micron scale
cluster in which the SiNP are well wrapped by SG and invariably
dispersed within the nanosheets matrix. FIG. 3b displays a higher
magnification HAADF-STEM image of the SG-Si electrode, while FIG.
3c displays the corresponding electron energy loss spectroscopy
(EELS) image (RBG mixed color mapping) of the highlighted area in
FIG. 3b. The pixels in the EELS image correspond to 3.4
nm.times.3.4 nm each. The yellow color is related to Si, while the
red color is sulfur (mixed red and yellow give orange with
different degrees relative to the concentration). It can be
inferred that sulfur follows the circumference of the SiNP. The
corresponding spectrum of the EELS based elemental mapping is shown
in the FIG. 4. It, again, confirms the presence of Si, S, N and C,
whereby S comes from the SG and N from the cyclized PAN (c-PAN). In
order to show how the binder PAN has shelled the particles and
connected them, a zoomed HAADF-STEM, with the corresponding TEM,
are presented in FIGS. 3d and 3e, respectively. They clearly show
that the particles are interconnected and wrapped with graphene. A
closer image of HRTEM focusing on one particle (FIG. 3f) shows the
crystalline Si particles with a shell of c-PAN and graphene
nanosheets.
Raman spectra of a PAN film deposited on copper foil, then dried,
before and after SHT is shown in FIG. 5a. While no features appear
before SHT, two characteristic peaks at .about.1346 cm.sup.-1 and
.about.1605 cm.sup.-1 are observed after SHT. These peaks
correspond to the "D" and "G" bands from the structural defects and
disorder from sp.sup.3-carbon atoms and the plane vibration of the
sp.sup.2-carbon atoms in two-dimensional lattice of the c-PAN,
respectively. This result again confirms that cyclization of PAN is
associated with graphitized carbon. The same features appeared with
the electrode materials after subjecting them to SHT, FIG. 5b.
It is well established that sluggish heating can cyclize
PAN,.sup.9,10 whereby c-PAN can provide stabilization of electrode
structures. A small proportion of graphitic oxide (GO), .about.1%,
may be added as an oxidizing agent to promote cyclization of PAN.
The characteristic exothermic peak for PAN cyclization is shown by
differential scanning calorimetry (DSC) in the FIG. 6a, with the
results consistent with previous reports..sup.11,12 Upon treatment,
PAN loses about 20% of its mass as shown by TGA, with results
provided in FIG. 6b. The SHT treatment has modified the chemical
structure of the PAN causing cyclization. The cyclization process
is associated with changes in the nature of chemical binding of
nitrogen with an evidence of enriched pyridinic type nitrogen, as
shown by the XPS results presented in FIG. 6c with a shift of
binding energy of nitrogen from 399.88 to 398.38 eV..sup.13,14
After cyclization, PAN has a .pi.-conjugate structure that is
believed to lower the electronic and charge transfer resistances of
the electrode, as evidenced by the electrochemical impedance
spectroscopy shown in FIG. 7. After inspecting the HRTEM images
introduced in FIG. 3 and the EDX mapping in FIG. 8, it can be
proposed that, almost every SiNP is caged in a carbon shell of
c-PAN. It is also clearly observed that there is no agglomeration
of SiNP.
X-Ray Photoelectron Spectroscopy
The elemental analysis of the electrode material after being
subjected to SHT is determined by the XPS survey spectrum as shown
in FIG. 9a, confirming the existence of Si (40%), S (5%), C (40%),
N (11%) and O (4%), with all compositions given in wt %. It should
be pointed out that XPS provides high surface sensitivity with
analysis depth of about 8-10 nm. Therefore, this elemental
quantification is different from the expected values which estimate
Si as 60% and S as .about.0.5%. The spectra of C in FIG. 9b shows
several common peaks, the first one (1) centered at 284 eV
corresponds to sp.sup.2 hybridized graphitic type carbon. Peak (2),
centered at 284.8 eV, denotes the presence of sp.sup.3 bonded
carbon. Finally, peaks (3) and (4) are characteristic of oxygenated
carbon and peak (5) is related to Plasmon loss features..sup.15-17
The core-level spectra in FIG. 9c shows the typical elemental Si
peak (1) located at 99.4 eV, with the minor peaks at higher binding
energies (.about.103.4 eV) related to oxygenated silicon or silicon
bonded to sulfur..sup.18 FIG. 9d shows the core-level spectra of S
in pure SG, with the atomic % of S of .about.2.5%. The S.sub.2p
doublet corresponding to the sulfide (C--S--C) structure is
observed at 164.0 and 165.2 eV and labeled (1) and (2). These peak
locations are in good agreement with the reported S.sub.2p3/2 and
S.sub.2p1/2 spin orbit couplet..sup.19,1,2 The other minor peaks
labeled as (3) in FIG. 9d and located at higher binding energies
are attributed to oxygen bound to sulfur (--SOx)..sup.20 The
structure elucidation of SG using XPS were used as the base to
determine the basic SG cluster used for DFT calculations discussed
vide infra. It is important to note that sulfur doped the graphene
sheets homogeneously, both on the edges and in the basal planes.
This was evidenced by STEM-EDX and EELS mapping shown in. A set of
samples were prepared as shown below and analysed in order to
understand the covalent chemisorbed interactions that occur between
Si and S in SG. The four samples prepared are: (1) Elemental sulfur
microparticles, SiNP and PAN dispersed well in DMF, followed by
solvent removal; (2) Sample 1 annealed at 450.degree. C. (same as
the SHT process); (3) SG+PAN+SiNP, dispersed well in DMF, followed
by solvent removal; and (4) Sample 3 annealed at 450.degree. C.
(same as the SHT process). High resolution XPS spectra for all of
these samples was obtained and is shown in FIG. 9e. Sample 1 shows
the regular S2p orbital split (doublet at 163.98 and 165.08 eV).
Additionally, a very depressed broad peak is observed at average
168 eV which may be attributed to silicon loss Plasmon
resonance..sup.21,22 Plasmon loss peaks involve a strong
probability for loss of a quanta of energy due to electron
interaction with the photoelectron..sup.23 For Sample 2, some
sulfur is covalently interacting with silicon while the majority of
sulfur is lost after annealing due to sublimation (m.p.
.about.120.degree. C.). The XPS results correspondingly show a
greatly enhanced peak signal for the silicon loss Plasmon
resonance. SG instead of elemental sulfur was used in Samples 3 and
4. The XPS signals for both these samples also showed a strong peak
for silicon loss Plasmon resonance, indicating possible
interactions between the Si and S atoms even before the annealing
process. This feature did not change with annealing, indicating a
similarly strong interaction between the two elements in both
cases. While not wishing to be bound by theory, it is speculated
that the reason of the enhanced Plasmon loss which appeared in
samples 2-4 is attributed to the interaction of Si with S. The
morphology investigated by SEM and pore size distribution
investigated by BET were determined for the electrode before and
after the SHT process, shown in FIGS. 11 and 12, respectively. The
micron sized particles of SiNP dispersed on the sheets of SG and
capped with c-PAN are demonstrated. The results of BET analysis
also show that the electrode structure developed increased
nanoporosity through the SHT process.
Electrochemical Performance.
FIG. 13a presents the typical galvanostatic charge/discharge
profiles of the SG-Si based electrode tested at 0.1 A g.sup.-1
between 1.5 and 0.05 V. The observed plateau in the first discharge
curve represents alloying of crystalline silicon with
lithium..sup.24,25 The SG-Si delivers an initial discharge capacity
of 2865 mAh g.sup.-1, based on all masses of SG, c-PAN and Si, with
a high first cycle Coulombic efficiency of 86.2%. If not mentioned,
all reported capacities are based on the total mass of SG, c-PAN
and Si. The voltage profiles of the subsequent cycles show slightly
different behaviour, which is common for the lithiation process of
amorphous Si formed during the first cycle. It is noteworthy that
the areal charge capacity is about 3.35 mAh cm.sup.-2, which is
close to the performance targets for next generation high energy
dense lithium ion batteries..sup.26 FIG. 13b shows the cycling
stability of the SG-Si at 0.1 A g.sup.-1. A stable cyclability up
to 100 cycles can be obtained, with an average capacity of 2750 mAh
g.sup.-1 (.about.3.35 mAh cm.sup.-2). The average capacity relative
to the mass of Si was determined as 3360 mAhg.sup.-1 (.about.3.5
mAh cm.sup.-2). The charge storage behavior was also characterized
by cyclic voltammetry (CV). FIG. 13c shows the first 5 cycles of
the SG-Si electrode in a coin cell at a scan rate of 0.05 mV
s.sup.-1. In the cathodic scan, there are two distinctive peaks
appearing at 0.27 and 0.22 V vs Li/Li.sup.+, indicating the
formation of Li.sub.12Si.sub.7 and Li.sub.15Si.sub.4 phases,
respectively..sup.27,28 In the anodic direction, the corresponding
two peaks are located at 0.31 and 0.49 V, representing the
dealloying of Li.sub.xSi to Si. All anodic and cathodic peaks
become broader and stronger as a result of cycling, which is a
common feature attributed to the conversion of Si into an amorphous
phase during lithiation/delithiation. Similar features were
observed for a G-Si investigated for comparison as shown in FIG.
14. The rate capability of the SG-Si electrode is shown in FIG.
13d, revealing the excellent kinetics of the SG-Si electrode at
different currents up to 4 Ag.sup.-1, Moreover, the robust
structure enables a very stable cycling, where a capacity of
ca.1033 mAh g.sup.-1 can be maintained for 2275 cycles at a rate of
2 Ag.sup.-1. By comparison, a similar electrode structure prepared
by replacing SG with non-doped graphene gives an inferior rate
capability and cycling stability, as shown in FIG. 13e. The high
capacity of the G-Si persists only for 80 cycles, then fades
gradually, reaching .about.400 mAh g.sup.-1 after 800 cycles. Such
a capacity fading is mainly attributed to the degradation of the Si
structure, where the expansion and shrinkage of SiNP during cycling
leads to the separation from graphene scaffold, and subsequent loss
of conductivity and instability in the solid electrolyte interphase
(SEI) structure. The significantly different electrochemical
performances put a spotlight on the important role of sulfur in
binding the SiNP to the surface of SG, which encouraged us to
further investigate it using density functional theory (DFT)
calculations discussed below. As a reference, a coin cell made of a
SiNP/PAN electrode, fabricated using SiNP and PAN subjected to a
SHT, also shows poor rate performance. In addition, its cycle
stability persists for only 65 cycles and then degrades rapidly to
almost zero capacity (FIG. 13f). These results emphasize the
important role of the covalent binding between Si and SG to enable
the impressive performance. In all cases, SG-Si, G-Si, and even
just Si when fabricated using PAN and followed by our SHT treatment
persists for at least for 2275, 80, and 65 cycles, respectively. On
the other hand, a coin cell fabricated using the same SiNP (60%),
Super P (20%), and the traditional binder polyvinylidene fluoride
(PVDF) (20%) without any SHT treatment has degraded very rapidly,
as shown in FIG. 15. Since we considered the total mass of the
electrode during calculation of the capacity, it is important to
show the relative contribution of each of the electrode components.
FIG. 13g is a pie chart showing the relative percent contribution
of the capacity observed in FIG. 13d. The results is based on the
battery performance testing for SG, under similar conditions, which
shows average reversible capacity of 235 mAh g.sup.-1, and an
electrode coated with only PAN after SHT treatment, which gave an
average capacity of 18 mAh g.sup.-1 (see FIGS. 16a and 16b). To
investigate the specific role of cyclized PAN and SG, reference
cells were fabricated from SG-Si-PVDF and GO-Si-PAN, respectively.
The battery performance of these two cells decayed rapidly as shown
in FIG. 17 and FIG. 18. This emphasizes the synergy of the
SG-Si-c-PAN in enhancing the electrode stability and providing
stable cycling.
The volumetric capacity for the cell presented in FIG. 13b was
calculated and the result was plotted in FIG. 19. It reveals that
the SG-Si-c-PAN electrode is able to provide a reversible capacity
of .about.2350 mAh cm.sup.-3 for up to 100 cycles. Coin cells
fabricated using different electrode composition of 40:30:30
(Si-SG-PAN) were tested and the results were introduced in FIG. 20.
It reveals similar trend of stable cycling and improved rate
capability.
After cycling a coin cell for 2275 cycles (FIG. 13d), the cell was
disassembled and the SG-Si electrode was subjected to further
characterization. FIG. 21a shows a HAADF-STEM image of the
electrode structure and FIGS. 21b-d provide the corresponding
colored EELS mapping for the elements S, C, and Si, respectively
(each pixel is 3.4.times.3.4 nm). This characterization shows that
the Si, as a result of frequent cycling, is confined in the
wrinkles of SG, and capped with cyclized PAN, utilizing the
covalent interaction between Si, SG and N. The location of the SiNP
is associated with regions of high sulfur and carbon. It is clear
that the engineered nano-architecture of the electrode design along
with the covalent interaction occurring between Si an SG, prevented
agglomeration of Si and maintained stable reversible cycle
stability for 2275 cycles. The same electrode was mapped using EDX
for comparison and the result was presented in FIG. 22. It is
important to emphasize here that EELS provides a near atomic scale
resolution to depict the distribution of atoms throughout the
sample. EELS also has a high sensitivity for lighter elements,
explaining why the signals from both carbon and sulfur are clearly
distinguished. FIG. 21e presents conceptual design of the electrode
structure before and after frequent cycles of continuous
lithiation/delithiation. On the other hand, inspection of the
electrode of the cell based on G-Si-cPAN after being cycled under
the same conditions shown in FIG. 13e by STEM reveals that by
continuous cycling silicon reveals more agglomeration, FIG. 23.
This emphasizes the important role of SG, which prevents
agglomeration of silicon and maintains electrode stability over a
large number of cycles.
Density Functional Theory Calculations
The graphene surface was modeled using a hydrogenated graphene
cluster (C.sub.54H.sub.18), which is also referred to as H
passivated graphene (see FIG. 24). The optimized bonding distances
of C--C (1.42 .ANG.) and C--H (1.09 .ANG.) in this model are in
good agreement with that for bulk graphite..sup.29 Based on this H
passivated C.sub.54H.sub.18 cluster, and based on bonding
configuration elucidated by XPS presented in FIG. 9d, a structure
of sulfur-doped graphene (SG) is proposed. The optimized SG
structure with some key structural parameters is shown in FIG. 25.
It can be seen that the SG has a distorted configuration. In all
the calculations, all the atoms in the cluster were allowed to
relax.
In order to describe the interactions between the Si and graphene,
the bonding energies (BE) of Si were defined by equation (1):
BE=E.sub.Si-graphene-E.sub.graphene-E.sub.Si (1) where
E.sub.Si-graphene, E.sub.Si, and E.sub.graphene represent the
energies of the Si-bound to the graphene structure, the Si atom,
and the graphene structure, respectively.
Si adsorption on different sites of the SG was studied. The results
are compared with those obtained on undoped graphene. FIG. 26a
presents the configuration of the stable Si adsorption on graphene
(G-Si), with Si sitting at the bridge site with adsorption energy
of 0.45 eV. Two stable configurations for Si adsorption on sulfur
doped graphene were observed. The first is represented as SG-Si(A),
which reveal the bonding of Si to location (A), FIG. 26b. The
second represents binding to location (B) and represented as
SG-Si(B), FIG. 26c. In SG-Si(A), Si was found to bind to S and two
"saturated" C atoms (C.sub.7 and C.sub.8), with the corresponding
binding energy of -2.02 eV. On the other hand, at the second
position, SG-Si(B), Si binds to S and two C's at the defect sites
(C.sub.2 and C.sub.3) forming two Si--C and one Si--S bonds,
leading to a binding energy of -3.70 eV. The higher binding energy
in the latter case indicates Si would be more energetically
favorable to bind to the defect C.sub.2 and C.sub.3 atoms. The
results show that Si attached on SG structure has a much higher
binding energy than that on graphene (G-Si). This result provides a
possible explanation for the much longer cycle stability in SG-Si
than in G-Si. The binding energy of silicon cluster made of 9
silicon atoms to different defect configuration in SG, FIGS. 26d
and e, was also studied. As expected, the covalent interaction
occurs between only two of the silicon atoms in the cluster
adjacent to the S and defect in SG. The binding energy was found to
be dependent on the defect configuration. FIG. 27 shows the binding
configuration with smaller cluster of 4 Si atoms. The same cluster
binds to SG stronger than binding to defect free graphene.
Hirshfeld charge analysis was also conducted to evaluate the
stability of Si on G and SG. The calculated charge distribution
before and after the Si adsorption on G and SG are given in Table
1. The results show that Si has a positive charge after its
adsorption on G and SG, which indicates that there are electrons
flow from the Si atom to the graphene substrate upon Si adsorption.
However, the electron flow is more significant for Si adsorption on
SG than that on G, because Si deposited on SG has a larger positive
charge than that on G. Table 1 also shows that the C atoms that are
bonded with the Si atom in SG-Si, such as C.sub.7 and C.sub.8 in
SG-Si(A), C.sub.2 and C.sub.3 in SG-Si(B), have more negative
charges than in G-Si (C.sub.2 and C.sub.3). These observations
suggest that the bonding between Si and SG is stronger than that on
G, providing further support for the stability of Si on SG.
TABLE-US-00001 TABLE 1 Hirshfeld charges distribution before and
after Si adsorption. The charge was calculated for the indicated
atoms on graphene (G) and sulfur doped graphene (SG), atoms
labeling are indicated in FIG. 15 and 16. Si adsorption on G Si
adsorption on SG Atoms G G-Si SG SG-Si(A) SG-Si(B) C.sub.1 -0.001
-0.004 0.010 -0.006 -0.004 C.sub.2 -0.001 -0.028 0.003 -0.022
-0.113 C.sub.3 -0.001 -0.029 0.004 -0.013 -0.100 C.sub.4 (or
Si.sub.4) 0.120 -0.016 -0.001 -0.019 S.sub.5 0.093 0.214 0.206
C.sub.6 -0.016 -0.035 -0.024 C.sub.7 -0.003 -0.070 -0.013 C.sub.8
-0.009 -0.028 -0.006 Si.sub.9 0.190 0.145
To better understand the covalent synergy between Si and graphene
substrates, the projected density of states (PDOS) of the Si atom
over G and SG were calculated, based on the electron structure and
bonding. As shown in FIG. 28a, there is a harmonic 2p-2p overlaps
between the C.sub.1-2p and C.sub.2-2p states at the whole energy
level (from 0 to -10 eV) in SG, showing the strong interaction
between the two C atoms. However, for Si and C, the harmonic
overlap occurs only between Si.sub.4-2p and C.sub.2-2p at a narrow
energy level (-2.about.-4 eV), indicating a weak interaction
between Si.sub.4 and C.sub.1 atom. For SG-Si (B), a large overlap
between the C.sub.6-2p and S.sub.5-2p state was observed (see FIG.
28b), indicating a strong S--C bonding. FIG. 28c shows that, more
Si.sub.9-2p state is occupied in SG-Si (B) and well mixed with
C.sub.2-2p state at a much broader energy level (from -1 to -9 eV)
as compared with that in G-Si. Additionally, there is also a
harmonic overlap between Si.sub.4-2p and S.sub.5-2p state (see FIG.
28d). The analysis of the PDOS revealed that the covalent synergy
was mainly due to the mixing between the C-2p and Si-2p states and
the C.sub.2-Si.sub.9 bond is much stronger than the
C.sub.2-Si.sub.4 bonding in G-Si, which attributes to the
significantly improved cycle stability.
The mobility of the adsorbed Li atom was also studied. FIG. 29
shows the transition state along the diffusion pathway. It was
found that, for Li atom diffuses away from the aforementioned most
stable sites in G-Si, it needs to overcome an energy barrier of
0.75 eV, as shown in FIG. 29a. However, the study of Li surface
diffusion on SG-Si(B) cluster shows that Li diffusion proceeds with
a barrier of 0.53 eV, FIG. 29b, which is slightly lower than that
found on G-Si. This observation indicates that S-doped graphene
could boost the mobility for Li atoms on Si-SG interface, which
facilitate the charge transfer.
While not wishing to be bound by theory, it is believed that the
improved cycling stability and rate capability of the Si-SG-cPAN
electrode is attributed to the structurally stable
nano-architectured design. It is believed that several changes
occur in the electrode structure during SHT: (1) PAN is cyclized by
forming graphitized carbon with 6-membered ring structure hosting
the nitrogen atoms in pyridine-like assembly. (2) Silicon anchors
and covalently interacts with the sulfur atoms, the activated
carbon associated with nanoholes in SG, and nitrogen in the
cyclized PAN. (3) The reconstruction and atomic scale
architecturing of the electrode lead to a robust structure in which
the SiNP are protected by a scaffold of graphene nanosheets and a
web of cyclized PAN. The cyclized PAN (c-PAN) forms an effective
shielding around the SiNP, which are already anchored on SG through
covalent interactions as confirmed by DFT calculations. In
addition, c-PAN sticks between the SG nanosheets, providing a 3-D,
interconnected structure that enables enhanced conductivity and
material robustness, as shown schematically in FIG. 2d.
It can be noted that the SiNP, after 2275 repetitive expansion and
contraction cycles, fractured and pulverized into smaller
particles. However, those fractured Si particles are still confined
within the continuous channels of the c-PAN shell, which is
overlaid on SG and maintains the electrical connection between Si
and graphene. The synergy of the interactions among Si/SG/c-PAN
leads to excellent cycle efficiency and capacity retention. The
unique and elegant special arrangement in the 3D structure of the
electrode provided appropriately sized voids along with elasticity
which accommodated repetitive volume expansion and contraction.
This results in preserving electrode integrity and prevented
degradation. Furthermore, sandwiching SiNP which have been capped
with cyclized PAN, between SG nanosheets forms a laminated
structure with limited open channels this supresses the penetration
of the electrolyte into the bulk of the electrode and limits most
of the SEI formation to the surface. We believe the TEM (EELS)
images shown in FIG. 13 can provide some indirect evidence that
most of the SEI formed on the outside. If the SEI formed on Si
nanoparticles, one should be able to see a large amount of SEI
covering Si since it is difficult for the fractured SEI to come
out. Another possibility is that the SEI would preferentially be
formed on the defective areas in the graphene, which might prevent
solvent getting into the space inside. Most of SEI appears to form
on graphene surface, which is more stable comparing with those
formed on Si surface.
Based on the DFT model, the Si atom has covalent interactions with
a sulfur atom in SG and two adjacent carbon atoms. The equivalent
strength of this covalent interaction is similar to that of a
single covalent bond. This interaction may not involve the Si atom
reacting directly with sulfur to form either SiS or SiS.sub.2, as
this would require de-bonding of sulfur from within the graphene
matrix, and may result in electrode degradation. In the case of Si
clusters (to simulate nanoparticles), only a small portion of the
silicon atoms form this covalent interaction with the SG. It is
believed that this type of Si does not participate in alloy
formation with lithium; however provides an anchoring site for the
majority of Si atoms within the nanoparticle that are readily
available for alloying/dealloying, thereby contributing to the
observed capacity.
It can be seen that Si bonds more strongly to SG than on G. One
reason is the covalent interaction of Si atoms with the sulfur
atom. The second reason is because the increased charge density on
the defective (with nanoholes) carbon adjacent to sulfur. This
indicates a covalent synergy for the interaction between Si and SG
leading to a superior material electrochemical performance, which
has not been seen with Si-G. It is clearly shown that, even after
2275 cycles of charge/discharge, the amorphous SiNP re-organised
into channels of the cyclized PAN and the sulfur pathway on
graphene, as seen in FIG. 13.
Although the invention has been described with reference to certain
specific embodiments, various modifications thereof will be
apparent to those skilled in the art. Any examples provided herein
are included solely for the purpose of illustrating the invention
and are not intended to limit the invention in any way. Any
drawings provided herein are solely for the purpose of illustrating
various aspects of the invention and are not intended to be drawn
to scale or to limit the invention in any way. The scope of the
claims appended hereto should not be limited by the preferred
embodiments set forth in the above description, but should be given
the broadest interpretation consistent with the present
specification as a whole. The disclosures of all prior art recited
herein are incorporated herein by reference in their entirety.
REFERENCES
1. Higgins D, Hoque M A, Seo M H, Wang R, Hassan F, Choi J-Y,
Pritzker M, Yu A, Zhang J, Chen Z. Development and Simulation of
Sulfur-doped Graphene Supported Platinum with Exemplary Stability
and Activity Towards Oxygen Reduction. Adv. Funct. Mater. 24,
4325-4336 (2014). 2. Higgins D C, Hogue M A, Hassan F, Choi J-Y,
Kim B, Chen Z. Oxygen Reduction on Graphene-Carbon Nanotube
Composites Doped Sequentially with Nitrogen and Sulfur. ACS
Catalysis 4, 2734-2740 (2014). 3. Hogue M A, Hassan F M, Higgins D,
Choi J-Y, Pritzker M, Knights S, Ye S, Chen Z. Multigrain Platinum
Nanowires Consisting of Oriented Nanoparticles Anchored on
Sulfur-Doped Graphene as a Highly Active and Durable Oxygen
Reduction Electrocatalyst. Adv. Mater., 1229-1234 (2014). 4. te
Velde G, Baerends E J. Precise density-functional method for
periodic structures. Phys. Rev. B 44, 7888-7903 (1991). 5.
Wiesenekker G, Baerends E J. Quadratic integration over the
three-dimensional Brillouin zone. J. Phys.: Condens. Matter 3, 6721
(1991). 6. Grimme S, Antony J, Ehrlich S, Krieg H. A consistent and
accurate ab initio parametrization of density functional dispersion
correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 132,
154104 (2010). 7. Li Y, Chen Z. X H/.pi. (X=C, Si) Interactions in
Graphene and Silicene: Weak in Strength, Strong in Tuning Band
Structures. J. Phys. Chem. Lett. 4, 269-275 (2012). 8. Arabi A A,
Becke A D. Assessment of the PW86+PBE+XDM density functional on van
der Waals complexes at non-equilibrium geometries. J. Chem. Phys.
137, 014104 (2012). 9. Arbab S, Mirbaha H, Zeinolebadi A, Nourpanah
P. Indicators for evaluation of progress in thermal stabilization
reactions of polyacrylonitrile fibers. J. Appl. Polym. Sci. 131,
40343 (2014). 10. Korobeinyk A V, Whitby R L D, Mikhalovsky S V.
High temperature oxidative resistance of
polyacrylonitrile-methylmethacrylate copolymer powder converting to
a carbonized monolith. Eur. Polym. J. 48, 97-104 (2012). 11. Wang
Y-X, Wang C-G, Wu J-W, Jing M. High-temperature DSC study of
polyacrylonitrile precursors during their conversion to carbon
fibers. J. Appl. Polym. Sci. 106, 1787-1792 (2007). 12. Wangxi Z,
Jie L. Comparative study on preparing carbon fibers based on PAN
precursors with different comonomers. J. Wuhan Univ. Technol.-Mat.
Sci. Edit. 21, 26-28 (2006). 13. Wang H, Maiyalagan T, Wang X.
Review on Recent Progress in Nitrogen-Doped Graphene: Synthesis,
Characterization, and Its Potential Applications. ACS Catalysis 2,
781-794 (2012). 14. Takahagi T, Shimada I, Fukuhara M, Morita K,
Ishitani A. XPS studies on the chemical structure of the stabilized
polyacrylonitrile fiber in the carbon fiber production process. J.
Polym. Sci. Part A: Polym. Chem. 24, 3101-3107 (1986). 15. Choi C
H, Park S H, Woo S I. Heteroatom doped carbons prepared by the
pyrolysis of bio-derived amino acids as highly active catalysts for
oxygen electro-reduction reactions. Green Chem. 13, 406-412 (2011).
16. Wohlgemuth S-A, Vilela F, Titirici M-M, Antonietti M. A one-pot
hydrothermal synthesis of tunable dual heteroatom-doped carbon
microspheres. Green Chem. 14, 741-749 (2012). 17. Paraknowitsch J
P, Thomas A, Schmidt J. Microporous sulfur-doped carbon from
thienyl-based polymer network precursors. Chem. Commun. 47,
8283-8285 (2011). 18. Morgan W E, Van Wazer J R. Binding energy
shifts in the x-ray photoelectron spectra of a series of related
Group IVa compounds. J. Phys. Chem. 77, 964-969 (1973). 19. Yang S,
Zhi L, Tang K, Feng X, Maier J, Mullen K. Efficient Synthesis of
Heteroatom (N or S)-Doped Graphene Based on Ultrathin Graphene
Oxide-Porous Silica Sheets for Oxygen Reduction Reactions. Adv.
Funct. Mater. 22, 3634-3640 (2012). 20. Yang Z, Yao Z, Li G, Fang
G, Nie H, Liu Z, Zhou X, Chen Xa, Huang S. Sulfur-Doped Graphene as
an Efficient Metal-free Cathode Catalyst for Oxygen Reduction. ACS
Nano 6, 205-211 (2011). 21. Yubero F, Holgado J P, Barranco A,
Gonzalez-Elipe A R. Determination of surface nanostructure from
analysis of electron plasmon losses in XPS. Surf. Interface Anal.
34, 201-205 (2002). 22. Yu Y, Tang Z, Jiang Y, Wu K, Wang E.
Thickness dependence of the surface plasmon dispersion in ultrathin
aluminum films on silicon. Surf. Sci. 600, 4966-4971 (2006). 23.
Grosvenor A P, Biesinger M C, Smart RSC, McIntyre N S. New
interpretations of XPS spectra of nickel metal and oxides. Surf.
Sci. 600, 1771-1779 (2006). 24. Magasinski A, Dixon P, Hertzberg B,
Kvit A, Ayala J, Yushin G. High-performance lithium-ion anodes
using a hierarchical bottom-up approach. Nat. Mater. 9, 461-461
(2010). 25. Lee S W, McDowell M T, Berla L A, Nix W D, Cui Y.
Fracture of crystalline silicon nanopillars during electrochemical
lithium insertion. Proc. Natl. Acad. Sci. 109, 4080-4085 (2012).
26. Liu N, Lu Z D, Zhao J, McDowell M T, Lee H W, Zhao W T, Cui Y.
A pomegranate-inspired nanoscale design for large-volume-change
lithium battery anodes. Nat. Nanotechnol. 9, 187-192 (2014). 27.
Liu B, Soares P, Checkles C, Zhao Y, Yu G. Three-Dimensional
Hierarchical Ternary Nanostructures for High-Performance Li-Ion
Battery Anodes. Nano Lett. 13, 3414-3419 (2013). 28. Key B,
Morcrette M, Tarascon J-M, Grey C P. Pair Distribution Function
Analysis and Solid State NMR Studies of Silicon Electrodes for
Lithium Ion Batteries: Understanding the (De)lithiation Mechanisms.
J. Am. Chem. Soc. 133, 503-512 (2010). 29. Rochefort A, Salahub D
R, Avouris P. The effect of structural distortions on the
electronic structure of carbon nanotubes. Chem. Phys. Lett. 297,
45-50 (1998).
* * * * *